The present invention is directed to an edge-lit thin panel illuminator having a direction selective light diffusing surface suitable for high contrast front lighting of large surface area graphic displays. The method for fabrication of the direction selective light diffusing surface disclosed herein forms no part of this invention but is the subject of a separate application Ser. No. 106,951, now U.S. Pat. No. 4,285,889, filed by J. H. Parsons concurrently with the present application.
In many applications it is desirable to illuminate graphic displays such as maps, photographs, charts and the like under conditions where conventional front lighting is inappropriate. For example, military systems often require viewing of graphic displays under conditions of low ambient light levels such as occur in darkened radar display rooms or aircraft cockpits. For another example, in some display systems, graphics are overlaid with a transparent window which itself contains display nomenclature. In such a system, ambient lighting of the graphics can create specular reflections from the transparent overlay which obscure not only the underlying graphics but also the display symbols on the transparent overlay.
One present approach to avoiding these problems is simply to backlight a transparent graphic display with a translucent diffusing surface such as a ground glass screen behind which is located one or more light sources. While capable of providing good contrast over a large surface, one disadvantage is that opaque graphic displays can not be used with such a display device but must first be converted into transparencies, a time consuming and often expensive process. Furthermore, a relatively large depth dimension is required for mounting the light sources, thus resulting in a bulky device.
In another approach to solving these problems, a thin-panel front illuminator display is achieved by end lighting a wedge shaped transparent panel having polished opposing surfaces. The panel overlies and is in contact with a display surface to be illuminated. A viewer observes the display through the panel at a near normal angle to the display surface. With the light source located at the wide end of the wedge, the panel acts as a light guide confining some of the light by total internal reflection between the opposing non-parallel surfaces. A portion of this confined light escapes symmetrically from both surfaces of the panel at an angle to the surfaces which is a function of the wedge angle of the panel. For a selected small wedge angle, the light escaping from the surface facing the viewer is confined within a sufficiently small angle to the panel surface so as not to be seen by the viewer observing the display. The portion of light escaping from the surface adjacent the display illuminates and is diffused by the display surface. A portion of this diffused light then passes through the panel to reach the viewer. While providing a compact high contrast display, the increasing thickness of the wedge shaped panel with respect to its length creates problems of bulk and viewing parallax which seriously limits the size of the display which can be illuminated.
In another approach to illuminating graphics, end lighting of a transparent planar panel having a partially roughened lower surface is used to front light a graphics display which underlies and is adjacent to the roughened surface. Some of the light trapped between the opposed parallel surfaces of the panel is diffused by the roughened surface and is distributed uniformly in direction through both panel surfaces. Although the light escaping through the lower surface illuminates the graphics display, some of the light escaping through the top surface is directed to the viewer. The viewer thus sees a bright roughened surface overlaying the illuminated display with the unfortunate result that the display has an unsatifactorily low contrast.